This invention relates to the field of antennas, and particularly to the area of high impedance (xe2x80x9cHi-Zxe2x80x9d) surfaces and to dual band, or multiple frequency band antennas.
A high impedance (Hi-Z) surface is a ground plane which has been provided with a special texture that alters its electromagnetic properties. Important properties include the suppression of surface waves, in-phase reflection of electromagnetic waves, and the fact that thin antennas may be printed or otherwise formed directly on the Hi-Z surface.
An embodiment of a Hi-Z surface is the subject of a previously pending provisional application of D. Sievenpiper and E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d, U.S. provisional patent application Ser. No. 60/079,953, filed on Mar. 30, 1998. Several improvements have been described in recently filed U.S. patent applications, including Ser. No. 09/520,503 for xe2x80x9cA Polarization Converting Reflectorxe2x80x9d filed Mar. 8, 2000; 09/537,921 entitled xe2x80x9cAn End-Fire Antenna or Array on Surface with Tunable Impedancexe2x80x9d filed Mar. 29, 2000; and U.S. patent application Ser. No. 09/537,922 entitled xe2x80x9cAn Electronically Tunable Reflectorxe2x80x9d filed Mar. 29, 2000, the disclosures of all of which are hereby incorporated herein by this reference.
This invention relates to techniques that extend the usefulness a Hi-Z surface by providing it with multiple-band operation, while preserving the inherent symmetry of the structure. This is an important development because it will allow for thin antennas operating in multiple bands. For example, one antenna could cover both GPS bands (1.2 and 1.5 GHz). A single antenna could also cover both the PCs band at 1.9 GHz. and the unlicensed band at 2.4 GHz, which is becoming increasingly important for such platforms as Bluetooth, new portable phones, and satellite radio broadcasting.
The present invention permits multiple band antennas to be much thinner than an ordinary Hi-Z surface having the same overall bandwidth, and also extends the maximum possible bandwidth of such surfaces by allowing them to have multiple high-impedance bands.
A high impedance (Hi-Z) surface consists of a flat sheet of metal covered by a periodic texture of metal plates which protrude slightly from the flat sheet. The Hi-Z surface is usually constructed as a two-layer or three-layer printed circuit board, in which the metal plates are printed on the top layers, and connected to the flat ground plane on the bottom layer by metal plated vias. One example of such a structure, consisting of a triangular lattice of hexagonal metal plates, is shown in FIG. 1 (the printed circuit boards are omitted in FIG. 1 for the sake of clarity in depicting the conductive elements). The metal plates have finite capacitance due to their proximity to their neighbors. They are linked by conducting paths which include the vias and the lower metal plate, and these paths contribute inductance. The result is a pattern of LC resonators, whose resonance frequency depends on the geometry of these elements. Each pair of adjacent metal plates in combination with their plated metal vias and the metal ground plane define a xe2x80x9ccellxe2x80x9d of the Hi-Z surface. A typical Hi-Z surface can have hundreds or even thousands of such cells.
The conventional high-impedance surface shown in FIG. 1 consists of an array of identical metal top plates or elements 10 disposed above a flat metal sheet or ground plane 12. It can be fabricated using printed circuit board technology with the metal plates or elements 10 formed on a top or first surface of a printed circuit board and a solid conducting ground or back plane 12 formed on a bottom or second surface of the printed circuit board. Vertical connections are formed as metal plated vias 14 in the printed circuit board, which connect the elements 10 with the underlying ground plane 12. The vias 14 are centered on elements 10. The metal members, comprising the top plates 10 and the vias 14, are arranged in a two-dimensional lattice of cells, and can be visualized as mushroom-shaped or thumbtack-shaped members protruding slightly from the flat metal surface 12. The thickness of the structure, which is controlled by the thickness of substrate 16, which is preferably provided by a printed circuit board, is much less than one wavelength xcex for the frequencies of interest. The sizes of the elements 10 are also kept less than one wavelength xcex for the frequencies of interest. The printed circuit board 16 is not shown in FIG. 1 for ease of illustration, but it can be readily seen in FIG. 2a. A large number of metal top plates may be utilized in forming a Hi-Z surface and only a small portion of the array of top plates 10 is shown in FIG. 1 for ease of illustration.
This structure has two important properties. It can suppress surface waves from propagating across the ground plane, and it provides a high surface impedance, which allows antennas to lie flat against it without being shorted out. However, these two properties only occur over a particular frequency band. The frequency and bandwidth of the high impedance region can be tuned by varying the capacitance and the inductance of the surface. The inductance depends on the thickness, which directly determines the bandwidth. The bandwidth is equal to 2xcfx80t/xcex, where t is the thickness, and xcex is the wavelength at resonance. For structures operating in the range of tens of GHz, a few millimeters of thickness provides bandwidth approaching an octave. However, for the important frequency regimes of S-band, and L-band, this thickness provides a bandwidth of only 10-20%. For UHF frequencies, several centimeters of thickness t provide no more than a few percent bandwidth.
Multiple band antennas often do not need to cover the entire frequency range spanning all bands of interest. However, with a multiple band Hi-Z surface such as that described herein, it is possible to cover several narrow bands that are separated by relatively wide bands of unused frequencies. In fact, this may be advantageous for suppressing out-of-band interference. For multiple band antennas, it is desirable to have a surface which provides a high impedance condition in multiple bands, where the bandwidth of each individual band is much less than the total frequency separation between them. This results in a thinner structure than one designed to cover all bands simultaneously, and can also suppress reception in other undesired signals. This is illustrated by FIGS. 2a and 2b. FIG. 2a shows a conventional two layer Hi-Z surface 1 with a relatively thick dielectric substrate 16. FIG. 2a-1 is a diagram of the single band gaps afforded by the Hi-Z surface of FIG. 2a. FIG. 2b shows an embodiment of a Hi-Z surface according to the present invention. FIG. 2b-1 is a diagram of the two band gaps afforded by the Hi-Z surface of FIG. 2b. The combined thickness of the two substrates 16 and 22 of the embodiment of FIG. 2b is less than the thickness of substrate 16 typically used in the prior art with Hi-Z surfaces.
Since the dual band embodiment of FIG. 2b has two bands each of which has a relatively small bandwidth compared to the embodiment of FIG. 2a, the dual band Hi-Z surface of FIG. 2b can be significantly thinner than the prior art structure of FIG. 2a. Thus dual band Hi-Z surface is both thinner than a comparable prior art surface, it is also better at suppressing out-of-band interference.
Techniques for producing multiple band Hi-Z surfaces might be summarized as providing multiple resonant structures in which local asymmetry splits a single mode into multiple modes, so that different internal regions of the Hi-Z surface can be identified with each distinct resonance. An important feature of these multiple band Hi-Z surfaces is that they are able to retain the same degree of overall symmetry as a traditional, single-band Hi-Z surface, although often with a larger unit cell size. This can be important because it has been found experimentally that conventional Hi-Z surfaces with at least threefold rotational symmetry allow a surface-mounted antenna to have any desired orientation without affecting the properties of the received or transmitted wave. Thus, using symmetrical structures simplifies the design of certain types of antennas, such as beam-switched diversity antennas. Conversely, if polarization control or adjustment is desired, the symmetry of the surface can also be broken, as is described U.S. patent application Ser. No. 09/520,503 noted above. This may be useful, for example, to allow conversion between linear and circular polarization. The present invention can be used with both symmetrical Hi-Z structures and with non-symmetrical Hi-Z structures.
In one aspect the present invention provides a high impedance surface having a reflection phase of zero in multiple frequency bands, the high impedance surface comprising: a ground plane; a plurality of conductive plates disposed in a first array spaced a distance from the ground plane, the distance being less than a wavelength of the radio frequency beam, said first array having a first lattice constant; and a plurality of conductive elements associated with said plurality of conductive plates, said plurality of conductive elements defining a second array, said second array having a lattice constant which can be the same as, or different than, the lattice constant of the first array.
The plurality of conductive elements can be provided by another array of conductive plates and/or by an array of conductive members which couple the plurality of conductive plates disposed in a first array to the ground plane.
In another aspect the present invention provides a method of making a high impedance surface exhibit a zero phase response at multiple frequencies, the method comprising the steps of: defining a high impedance surface having a ground plane and a plurality of conductive plates disposed in a first array spaced a distance from the ground plane, the distance being less than a wavelength of the radio frequency beam, defining a plurality of conductive elements associated with said plurality of conductive plates, said plurality of conductive elements connecting said plurality of conductive plates to said ground plane; and locating each of said plurality of conductive elements spaced a distance from a geometric center of an associated conductive plate and with all conductive elements associated with predetermined clusters of conductive plates being spaced in a direction pointing towards a common point for a given cluster.